This invention relates to a phase shift mask, especially a halftone phase shift mask, and a process for preparing the same.
It is now required to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices. The photo mask used in the pattern formation is also required to be of finer definition. A number of phase shift masks have been developed to meet the demand. Among others, halftone phase shift masks which are relatively easy to prepare are utilized in practice. Referring to FIG. 4, the principle of the halftone phase shift mask is described. A shifter 42 for shifting light phase is formed on a quartz substrate 41. When light is transmitted by this structure, interference occurs between the light transmitted by the shifter 42 and having phase shifted and the light transmitted by only the substrate and having the original phase. Resolving power is improved by utilizing this interference. The phase should be shifted accurately 180 degrees. In the manufacture of the mask, it is necessary to minimize the generation of particles and defects. If the size of particles or defects is approximate to the micro mask pattern, a pattern associated with defects can be added to the desired mask pattern, lowering the yield of mask formation. This eventually lowers the yield of semiconductor circuit fabrication.
In the prior art, molybdenum silicide materials are practically used as the shifter material for halftone phase shift masks. However, they have the following drawbacks.
The molybdenum silicide material of which the shifter film of conventional halftone phase shift masks is formed is very difficult to deposit as a film. This leads to a marked decline of manufacturing yield at the stage of photomask blanks prior to the stage of completed masks.
While there are speculated several causes of this problem, the predominant causes are (1) reactive sputtering and (2) a relatively low target density.
(1) Reactive sputtering
In forming the MoSi shifter material, a sintered body of Mo and Si represented by MoSix wherein x is 2 to 3 is most often used as the target. Upon sputtering, a large volume of oxygen, nitrogen or methane is fed as the reactive gas. However, since these gases are highly reactive, gas components can be taken in the film being deposited before the gas reaches the center of the target. As a result, a significant difference of quality is introduced in the film in a direction toward the target center. This film quality difference appearing as a film quality distribution of shifter material invites a significant change of refractive index. In addition, particles can grow in the vapor phase within the plasma, resulting in the shifter film having many defects.
The generation of particles and defects can be suppressed by reducing the flow rate of the reactive gas. However, if the flow rate of the reactive gas is excessively reduced, the transmittance of the film cannot be increased to the desired level.
(2) Relatively low target density
The other drawback of the prior art MoSi halftone phase shift mask is that the target in the form of a sintered body of molybdenum metal and silicon metal cannot be fully consolidated and becomes a bulky target having a lower density than the desired level. When such a target having a low density is sputtered, there often generate particles and defects. Additionally, the target surface is susceptible to corrosion and oxidation by the reactive gas, with the increased tendency of generating defects.
While these problems are considered from the standpoint of defects in the shifter film, the prior art phase shift mask has another serious problem associated with an uneven distribution of film quality.
The reason is that the reactive sputtering requires to take many components from the reactive gas into the film. The uneven distribution of film quality suggests that the phase shift angle expressed by the following formula (1) has a distribution within the mask substrate. It is then quite difficult to form a phase shift mask with a high precision.
D=xcex/2(nxe2x88x921)xe2x80x83xe2x80x83(1)
Herein D is the thickness of a shifter film for 180 degree phase shift, n is the refractive index of the shifter material, and xcex is a transmission wavelength.
As mentioned above, the prior art process of depositing the shifter material in film form entails the generation of particles causing film defects. It is very difficult to deposit a defect-free shifter film.
Furthermore, the reactive sputtering uses a large volume of reactive gas which causes the generation of particles. Since the manner of taking in the reactive gas is variant in a direction toward the center of the target, it is difficult to control the process so as to form a homogeneous film over the entire substrate surface. It is also a problem from the standpoints of defect prevention and film quality uniformity that the target material is limited to a sintered body of MoSi.
An object of the invention is to provide a phase shift mask which has solved the above-mentioned problems of the prior art halftone phase shift masks and complies with the drive for miniaturization and higher integration in semiconductor integrated circuits. Another object is to provide a process for preparing the phase shift mask.
It has been found that a phase shifter film formed of gadolinium gallium garnet is effective to the above object, and that the gadolinium gallium garnet film can be effectively formed by sputtering an oxide single crystal of gadolinium gallium garnet.
In a first aspect, the invention provides a phase shift mask comprising a phase shifter formed on a substrate which is transmissive to exposure light. The area of the substrate which is not covered with the phase shifter serves as a first light transmissive region and the phase shifter serves as a second light transmissive region. The phase shifter is constructed of gadolinium gallium garnet. Preferably, the phase shifter shifts the phase of transmitted exposure light by 180xc2x15xc2x0 and has a transmittance of 3 to 30%.
In a second aspect, the invention provides a process for preparing a phase shift mask comprising the steps of forming a film of gadolinium gallium garnet on a substrate which is transmissive to exposure light, by sputtering; forming a resist pattern on the gadolinium gallium garnet film; and dry etching the gadolinium gallium garnet film through the resist pattern to pattern the gadolinium gallium garnet film. Preferably, the sputtering step uses an oxide single crystal of gadolinium gallium garnet as the target. Also preferably, the sputtering step is reactive sputtering using a mixture of an inert gas with an element source gas selected from among oxygen, nitrogen and carbon. More preferably the element source gas is selected so as to provide a flow rate of 1 to 25% of oxygen, 2 to 20% of nitrogen or 2 to 15% of carbon, based on the flow rate of the inert gas.
To produce the phase shifter material, according to the invention, sputtering is carried out using an oxide single crystal whose composition is uniform on the atomic level as the target material rather than the bulky sintered body, typically of MoSi, used in the prior art. The sputtering of the oxide single crystal minimizes the generation of particles from the target and enables to form a homogeneous film over the entire substrate surface. Since the target composition contains a quantitative amount of oxygen, the oxygen component to be taken into the sputtered film from the reactive gas is limited. Then the reactive gas may be fed in a small amount just necessary for the fine adjustment of a transmittance and a phase shift angle. This permits easy control of the composition of the shifter material and minimizes the growth of particles in the vapor phase within the plasma, ensuring that a shifter film is formed defect-free.
As understood from formula (1), the gadolinium gallium garnet film provides a 180 degree phase shift of transmitted light at a relatively small thickness since it has a high refractive index. This minimizes the influence (mainly on focal depth) of the shifter film thickness upon exposure.